1. Field of the Invention
This invention relates to the field of electronics amplifiers and more particularly to the field of signal conditioning circuits for signal compression and expansion such as those used in audio entertainment systems.
As an example of signal compression, the magnetic tape on a tape deck might have a dynamic range of only about 60 Db. A signal source might be supplying the tape with a signal that has a dynamic range of about 80 Db. If the signal level of the source material is adjusted such that the largest signals are not clipped, the resulting recording would have passages that are very soft and possibly inaudible when reproduced. To hear the softer passages when replayed at a later time, the output level would have to be increased to a level that might begin to reproduce the noise floor of the recording media on the tape.
A compressor circuit is used to compress the 80 dB source signal range into about 40 Db by attenuating the larger amplitude signals and not the lower amplitude signals. The signal source output signal amplitude is then adjusted to place the topmost value of the 40 dB signal range out of the signal source at a level that matches the top value of the 60 dB signal range of the tape in the recorder. The signal range on the tape is thereby positioned at levels between the top limit of the dynamic range of the tape, absent clipping, down 40 dB leaving a remaining 20 dB of signal dynamic range that contains noise but which is largely absent signal information.
The large amplitude low frequency signals are compressed in amplitude along with bursts or instantaneous transients that are typically high frequency information. A compressor appears to function to attenuate only the larger amplitude signals leaving the lower amplitude signals unaffected. The larger the range of change, the larger the attenuation.
This invention combines a multichannel pre-amplifier with automatic balancing on the mid-range and high frequency channels with automated bandwidth expansion and compression as a function of signal amplitude. The invention therefore functions as a simplified compander in providing the features of both a compressor and an expander in the same topology.
2. Description of Related Art
U.S. Pat. No. 3,789,143 issued Jan. 29, 1974 to D. E. Blackmeer teaches a "Compander With Control Signal Logarithmically Related To The Instantaneous RMS Value Of The Input Signal" that shows a circuit in which the gain control is derived proportional to the RMS value of the audio signal that is received at its input. This circuit topology does not appear to contain a bandwidth sensitive feature that reduces the bandwidth of the amplifier in response to receiving signals of lower amplitude nor does it show the circuit in combination with appropriate pre-amplification characterized to provide an automatic balance between the amplitude of the midrange and high frequency ranges of signals being processed by the pre-amplifier.
U.S. Pat. No. 4,482,866 issued on Nov. 13, 1984 for a Reference Load Amplifier Correction System, and U.S. Pat. No. 4,638,258 issued on Jan. 20, 1987 for a Reference Load Amplifier Correction System, both issuing to Robert C. Crooks. Both of these patents have a common assignee Barcus-Berry Electronics, Inc. of Huntington Beach, Calif. now owned by BBE Sound, also of Huntington Beach, Calif., the assignee of this application. The `866` patent and the `258` patent show circuit topologies that can be used as the pre-amplifier section of the claimed combination invention; however, the arrangement of FIGS. 8 and 9 in the `258` patent is preferred. None of the aforementioned references show or teach the claimed elements of a filter channel or the absolute value circuit in combination with the pre-amplifier.
A first object of the invention is to provide an amplifier circuit that has: a pre-amplifier with automatic gain control circuitry for balancing the high and mid range signal components to provide a compensated signal. The compensated signal is coupled to a first channel.
The first channel is a filter circuit that is coupled to receive the compensated signal, and a control signal. The filter circuit provides a modified compensated signal, and an output signal. The bandwidth of the filter circuit is adjusted automatically by the control signal. As the amplitude of the control signal is reduced, the bandwidth of the filter circuit is reduced.
The second channel is an absolute value circuit that responds to the modified compensated signal and provides a control signal proportional to the amplitude of the modified compensated signal.
A second object of the invention is to provide a pre-amplifier that has a high frequency correction channel, a low frequency correction channel, a mid range channel, an automatic adjusting means and a summing circuit.
The high frequency correction channel, the low frequency correction channel and the mid range channel each receive the input signal referred to as a program signal from a source such as a record or tape player.
The high frequency correction channel provides a high frequency compensated signal; the low frequency correction channel provides a low frequency compensated signal and the mid range channel provides a mid range signal. The automatic adjusting means for automatically adjusts the balance between the high frequency compensated signal and the mid range signal. The summing circuit adds the high frequency compensated signal, the low frequency compensated signal and the mid range signal to provide the compensated signal. In a preferred embodiment, the mid range channel has an inverting amplifier.
It is a second object of the invention to provide an embodiment of the absolute value circuit that has a high pass amplifier coupled to receive the modified compensated signal from the filter circuit. The high pass amplifier amplifies the modified compensated signal couples its output, a first amplified signal to the input of an absolute value rectifier circuit. The absolute value rectifier circuit rectifies the first amplified signal and provides an absolute value signal. A peak detection circuit receives the absolute value signal and provides a peaked absolute value signal. A low-pass filter and bias circuit receives the peak absolute value signal and provides the control signal to the filter channel. The low-pass filter and bias circuit has a dc bias adjustment control for adding a dc bias level to the control signal.
It is a third object of the invention to provide an embodiment of the filter circuit that has an input amplifier for summing the compensated signal and an integrator output signal and that provides the modified compensated signal to the input of the absolute value circuit and to the input of a voltage controlled amplifier. The voltage control amplifier has a second input coupled to receive the control voltage. The voltage control amplifier provides an integrator input signal current to the input of an integrator circuit.
The integrator circuit integrates the integrator input signal current and provides an integrator output signal. A low-pass active filter receives the integrator output signal and filters it to provide the output signal.
In another alternative embodiment of the filter circuit, the input amplifier for summing the compensated signal and the integrator output signal provides the modified compensated signal to a photocell that has a photosensitive resistor and a light emitting diode driven by the control signal from the output of the absolute value circuit. The photosensitive resistor responds to light from the light emitting diode to modify the modified compensated signal to be an integrator input signal. The photosensitive resistor couples the integrator input signal to the integrator input.